knjiga solarna energija dobra

UNDERSTANDING a-Si pin CELLS 533
neglect other aspects of power loss in the cell, which apply when field collapse may be
neglected [127, 128], in particular for lower intensities.
12.4.3 Absorber Layer Design of a pin Solar Cell
In this section we address the issues that determine the absorber (or “intrinsic”) layer
thickness. Figure 12.17 illustrates a computer calculation showing how the output power
of an a-Si-based pin cell varies with intrinsic layer thickness. The differing curves represent
results for monochromatic illumination using varying photon energies with the
specified absorption coefficients. All the curves were calculated for the same photon flux.
Such illumination conditions might be achieved experimentally using a laser whose photon
energy could be tuned from 1.8 to 2.3 eV; sunlight, of course, presents a much more
complex situation, as we discuss in Section 12.4.6.
We first discuss results for illumination through the p-layer (solid symbols in the
figure). For intrinsic layers that are sufficiently thin, the power is proportional to the
number of photons absorbed (i.e. to the product of the thickness d and the absorption
coefficient α). In this limit the fill factors have nearly ideal values around 0.8.
As the thickness of the cell increases, the power saturates. First consider the
behavior for strongly absorbed illumination (α = 100 000/cm – corresponding to a photon
energy of about 2.3 eV in Figure 12.2). Power saturation occurs for thickness greater than
100 nm, which is the typical distance at which the photons are absorbed. Since thicker
cells do not absorb much additional light, the power stops increasing past this length.
20 100 000/cm
15
Power
[mW/cm 2 ]
10
5
0
50 000/cm
20 000/cm
10 000/cm
5000/cm
0 100 200 300 400 500
Intrinsic layer thickness
[nm]
Figure 12.17 Computer calculation of the power output from a pin solar cell as a function of
intrinsic layer thickness. The differing curves indicate results for monochromatic illumination with
absorption coefficients from 5000/cm to 100 000/cm; for typical a-Si:H, this range corresponds to
a photon energy range from 1.8 to 2.5 eV (cf. Figure 12.2). Solid symbols indicate illumination
through the p-layer and open symbols indicate illumination through the n-layer. Incident photon
flux 2 × 10 17 /cm 2 s; no back reflector